With the growing maturity of mobile telecommunication technologies, demands of mobile subscribers increase rapidly for services such as Internet, multimedia application, file transfer and etc, one of whose notable features lies in different requirements for the uplink capacity and the downlink capacity. Thereafter, telecommunication system needs to be able to allocate different capacities for the uplink and the downlink, according to different requirements of services.
CDMA-TDD technology can nicely meet the requirements of these services, because the utilization of the TDD frequency bands between its uplink and downlink is not fixed, but can be adjusted according to the requirements of services. Therefore, CDMA-TDD technology finds its niche in the mobile telecommunication field where CDMA-FDD technology dominates. For instance, it's covered in the 3rd generation UTRA (UTRA: UMTS Terrestrial Radio Access) standard.
In CDMA-TDD technology, all subscribers send messages concurrently using the same band, which is likely to cause mutual interference. Therefore, multiple access interference arises as its major capacity bottleneck, and thus the key issue to increase system capacity is to reduce multiple access interference.
Researches and practices indicate that power control, especially uplink power control, can effectively remove the near-far effect and reduce various interferences, and thus enlarge system capacity and provide better QoS. Two kinds of existing power control, open-loop power control and closed-loop power control, are both widely employed in our daily life. However, undeniably their working principles both have disadvantages, which should be overcome to boost the system performance.
Now, we will take the uplink power control of UTRA TDD as an example, to demonstrate the principles and flaws of existing open-loop power control and closed-loop power control mechanisms.
FIG. 1 illustrates a simplified CDMA system, which consists of n transmitters 1, n channels 3 and a receiver 2. As shown in FIG. 1, the receiver 2 can receive signals from different transmitters 1, but only one signal is useful while others will be deemed as interference.
In open-loop power control mechanism, the output power of a mobile terminal is based on the path loss of the downlink PCCPCH (Primary Common Control Physical Channel), the target Signal-to-Interference Ratio (SIR) provided by the outer loop and the interference signal level in every time slot of the telecommunication network. The following formula is to calculate the uplink transmit power (please refer to 3GPP TS25.331: RRC Protocol Specification):PUE=αLPCCPCH+(1−α)L0+IBTS+SIRtarget+C   (1)Where,                PUE is the transmit power level of the mobile terminal in dBm;        LPCCPCH is the measured path loss in dB on PCCPCH;        L0 is the long term average of LPCCPCH in dB;        IBTS is the interference signal power level at the base station in dBm, which is broadcast on BCCH (Broadcast Control Channel) or individually signaled to each mobile terminal.        α is a weighting parameter, which represents the quality of the path loss measurements. α may be a function of the time delay between the uplink time slot and the latest downlink PCCPCH time slot.        SIRtarget is target SIR provided by the outer loop, which is individually signaled by the base station to every mobile terminal.        C is a constant value to be set by a higher layer.        
The above describes the basic principle of open-loop power control, while closed-loop power control is based on SIR and TPC (transmit power control) processing procedure (please refer to 3GPP TS 25.224: Physical layer procedures (TDD)). The base station will first estimate the Signal-to-Interference Ratio (SIRest) of the uplink channel, and then generate TPC command and transmit the command to the corresponding mobile terminal according to the following rule: if SIRest>SIRtarget, then the TPC command to transmit is ‘down’; if SIRest<SIRtarget, then the TPC command to transmit is ‘up’. SIRtarget is the target SIR, to be set by a higher-level outer loop.
After the TPC command is sent to the mobile terminal, the processing procedure needs to make relevant decisions according to the TPC bits. When the TPC bits are regarded as ‘down’, the transmit power of the mobile terminal will be decreased by one power control step; when the TPC bits are regarded as ‘up’, the transmit power of the mobile terminal will be increased by one power control step.
For example, the coordinate operation of open-loop power control and closed-loop power control can be an uplink signal transmission on DTCH (Dedicated Traffic Channel). The initial uplink transmit power of DTCH can be determined by open-loop power control. After the initial phase, closed-loop power control starts to take effects to control the transmit power, along with the open-loop power control.
However, current uplink open-loop power control and closed-loop power control both have their disadvantages. The interference signal level used in current uplink power control algorithms is based on previous status information of the communication link, and the base station's periodical adjustment to the transmit power of each mobile terminal is done independently. So, power adjustment of every mobile terminal can affect SIR of others, causing constant variances of the interference signal level. This is especially true at the beginning of a call and after each handover. Therefore, power control of mobile terminal is very slow for convergence speed, which degrades the system performance.